The Bush administration's push to deploy a $22 billion missile defense system by this time next year could lead to unforeseen cost increases and technical failures that will have to be fixed before it can hope to stop enemy warheads, Congressional investigators said yesterday.

The General Accounting Office, in a 40-page report, said the Pentagon was combining 10 crucial technologies into a missile defense system without knowing if they can handle the task, often described as trying to hit a bullet with a bullet.

The report especially criticized plans to adapt an early warning radar system in Alaska to the more demanding job of tracking enemy missiles, saying it had not been adequately tested for that role.

The overall uncertainty, the investigators said, has produced "a greater likelihood that critical technologies will not work as intended in planned flight tests." If failures ensue, they added, the Pentagon "may have to spend additional funds in an attempt to identify and correct problems by September 2004 or accept a less capable system."

Dr. Philip E. Coyle III, a former head of weapons testing at the Pentagon, said in an interview that the report showed that if the system was switched on in late 2004, it would be "no more than a scarecrow, not a real defense."

Some critics say the timetable is devised to field a missile defense system before the 2004 election so President Bush can point to it as a fulfilled campaign pledge.

But Pentagon officials say that the timing is prompted by security concerns and that the Sept. 30, 2004, target date came about simply because it is the end of the fiscal year.

The system, initially with six rocket interceptors in Alaska and four in California, will fire "kill vehicles" that would destroy warheads by force of impact.

In its report, the accounting office said the Pentagon expected to spend $21.8 billion on the system between 1997 and 2009.